Foamed polypropylene filaments



Jan. 21, 1964 F. R. CRAMTON FOAMED POLYPROPYLENE FILAMENTS 2Sheets-Sheet 1 Filed March 13, 1963 m m m FRANK R. CRAMTON ATTORNEY Jan.21, 1964 F. R. CRAMTON FOAMED POLYPROPYLENE FILAMENTS 2 Sheets-Sheet 2Filed March 13, 1963 m mlh INVENTOR I FRANK R. CRAMTON BY 6M 9M ATTORNEYUnited States Patent sci or. 15 -159) stretching the filaments toincrease the molecular orientation along the fiber axis. The resultantstretch oriented rlaments are character zed by increased tensilestrength, ow elongation, and increased stifiness and resiliency.

in the melt extrusion of polypropylene filaments, it is necessary tosolidify the fdaments by cooling prior to softening for stretchorientation such as by quenchlng in a non-sol ent liquid. Subsequently,the filaments are heat softened and longitudinally stretched to orientthem. In stretch orienting heat softened filaments of polypropylene, thefilament is often supported by some solid surface after it has beensoftened, but before it is stretched. For examole, the filament may bepassed through a heating zone wherein the filaments are heated to atemperature within their softening range by the action of a hot liquidor a hot gas in the heating zone. in order to expose the polypropylenefilaments to the fluid source of heat for a period of time sufiicient toraise the temperature thereof to a point within its softening range, thefilaments are frequently conducted through a sinuous path within theheating zone by means of supporting rolls. Such rolls may themselves beindependently heated so that they will serve as the source of heat forheating the filaments. In such a case, the fluid source of heat in thezone may be dispensed with. When polypropylene filaments are passeddirectly through a heating zone unsupported by rolls or other surfaceswhile in the zone, there may be provided a device such as a rollassembly on the outside of the zone for snubbing the filaments afterthey have been heat softened but before they are stretched or drawn.

When a polypropylene filament having a circular cross section is cooledto solidify it subsequent to extrusion, the filament undergoes severecross sectional distortion. Thus, when an extruded polypropylenefilament having a circular cross section is cooled to solidify it priorto softening it for stretch ori station, the filament does not retainits circular cross sectional shape but acquires instead an elliptical ornearly elliptical cross sectional shape. This phenomenon is known as outof roundness. The degree of out of roundness (i.e., amount of crosssectional distortion) increases as the rate of cooling of the extrudedfilament increase Moreover, the out of roundness is accentuated when thefilament, after being heat softened, is supported by a solid surfaceprior to and/ or during the drawing thereof; The tension applied to thefilament during the stretching operation while the heat softenedfilament is supported on a solid surface is responsible for this sourceof out of roundness.

The out of roundness, or cross sectional distortion, may be sopronounced that the length of the major axis of the elliptical crosssectional shape of the distorted filament may be two or more timesgreater than the length of the minor axis thereof. This phenomenon maybest be illus- Patented Jan. 21', l

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trated by reference to the relative dimensions of the cross sectionalshape of the filament. The relative dimension of the cross sectionalshape of a filament or dimensional ratio, is the ratio of the length ofthe major axis to the length of the minor axis. Thus, if the crosssectional shape of the filament is substantially circular, both axes aresubstantially the same length and the relative dimension of the crosssection or dimensional ratio is substantially unity. However, when thelength of the major axis is twice as great as the length of the minoraxis, the relative dimension or dimensional ratio is 2.0.

When a non-foamed polypropylene filament is extruded and rapidly cooled,notable diameter variations, as well as crystalline irregularities inthe form of large vacuoles within the filament, are found to occur alongthe length of the filament; The large low strength vacuoles ap pear tobe a result of rapid thermal and crystallization contraction. Althoughthe thermal and crystallization contraction may be controlled to someextent by the use of controlled slow cooling of the filament af erextrusion, filaments possessing the best abrasionresistance are obtainedby rapid quenching or cooling. When stretch oriented, the low strengthvacuole points yield more than normal to the orientation stress andcreate extreme diameter variations resulting in points of low tensilestrength along the filament. The uniform small cells of the foamedfilaments of the present invention absorb the thermal andcrystallization contraction along the axis to the point where the largevacuoles no longer exist. The foamed filaments maintain a substantiallyround, uniform diameter throughout the length of the filament thussubstantially increasing the tensile strength.

The foregoing described disadvantages are avoided by the practice ofthis invention which, briefly, comprises produciug'a dimensionallystable filament consisting essentially of oriented isotactic foamedpolypropylene having an efiective blow-up of from 1% to about 15%, andhaving a cross section which is substantially round. Such a filament isproduced by preparing a mixture of isotactic polypropylene and aquantity of foaming agent sulfiient to produce from about 1% to 15%blow, introducing the mixture into a heating zone, melt extruding afilament from the mixture, quenching or otherwise solidifying'thefilament, subsequently softening the filamerit and longitudinallystretching said filament. Prior to and/or during the stretching of thefilament, it may be supported on a' solid surface.

The preferred polypropylene which may be used in the practice of thisinvention is iso-tactic polypropylene which is a high molecular weight(i.e., above about 45,- C6 0) solid polymer exhibiting a crystallineX-ray diffraction pattern. Such a polymer has a density between 0.9-3and 0.94 and a melting point above about 320 F. These polymers may beprepared by methods now well known in the art such as the proceduresdescribed by G. Natta in the Journal of Polymer Science, Vol. XVl, pp.143 to 154 (1955) and in US. Patents 2,882,263; 2,874,153 and 2,913,442.

The preferred foaming agents which may be used in the practice of thisinvention are those which will decompose at temperatures approximatingthe extrusion processing temperatureof'polypropylene. For instance,foaming agents-such as 1,1-azobisforrnamide (Kempore R425), 4,4 oxybis(benzenesulfonyl sem carbozide), trihydrazinosym.-triazine (THT),bis-benzenesulfonyl hydraeide EBSH) and barium' azodicarboxy late("Expandex 177) are typical of those which may be used in the practiceof this invention.

The foaming agentshor'rl'd be present in an amount sufiicient to produceup to 15% blow in the extruded Percent blow= It has been found thatfilaments containing up to about blow will give the desired propertiesof improved dimensional stability and reduced diameter variation with nosignificant loss of stiffness or abrasion resistance.

The polypropylene, which is preferably in particulate form, isthoroughly admixed with the blowing agent an the admixture is introducedinto a heating zone. In the heating zone, the polypropylene is heated toa suitable extrusion temperature above its melting temperature. Afterthe admixture has been heated to said temperature, it may be maintainedin gastight environs until it has been extruded to prevent prematureblowing. The melt of polypropylene is then extruded into a filament.After the filament has been extruded, it may be solified as by quenchingin a non-solvent bath or by air cooling. Subsequently, it is heatsoftened by some suitable means. This may be accomplished by conductingit through a heating chamber wherein it is supported, preferably, by aplurality of rolls or equivalent supporting members, the peripheralsurface of each providing a supporting surface. The filament may beheated while in the heating zone by means of a fluid source of heat,either gas or liquid, or by means of independently heated rolls.Alternatively, the supporting rolls may be eliminated from the heatingzone and the filament passed directly through the zone which is heatedby some fluid source of heat, or by radiant heat, the filament beingunsupported by any member. The softened filament may be snubbed by meansof a suitable device such as a conventional snubbing roll assembly afterit leaves the heating chamber but prior to the drawing operation. Thus,the heat softened filament may be supported by the surface of thesnubbing device after it leaves the heating zone and while in a softenedcondition.

After the filament has been softened, it is stretched longitudinally toincrease the molecular orientation along the fiber axis. Any amount ofstretching will increase the molecular orientation. However, maximumbenefits are attained by stretching the filaments from about 6 to about11 or more times of their length.

The drawn filaments of foamed polypropylene have diameters which rangefrom 10 to 500 mils and preferably from 75 to 259 mils.

The cross sectional shape of the drawn filament prepared according tothis invention is substantially the same as that of the undrawn filamentafter it is extruded but before it is solidified and subsequently heatsoftened prior to the drawing thereof. That is the relative distancebetween the midpoint of the cross section and the periphery, or therelative dimensions, remain approximately or nearly the same so that,although the size or area of the cross section are considerably lessafter drawing, the round configuration remains substantially the same.

The filament or bristle produced according to this invention has astructure such that a major part of the foamed portion is located at thecenter of the bristle and extends lengthwise therethrough. Since thefoamed portion is located at approximately the center area of thefilament, the thermal and crystallization contraction, which occursprimarily at this center area is apparently absorbed by the foam,resulting in a rounder filament. Substantially none of the foam is inthe outer part. Thus the outer portion of the bristle is continuous andnonporous after extrusion. ther polymers, such as polyethylene, must bedrawn in order to obtain a continuous surface.

Uniformly shaped filaments produced by the process of this invention areextremely useful in the production of uniform tufts of bristles in abrush. This results in a tmore uniform distribution of wear of thebristles. Moreover, such tufts are more easily mounted in brush heads.Flex action in any direction is much more nearly uniform than in ovalfilaments-ie, filaments which have gone out of round during processing.Also, the foamed filements of this invention possess better flaggabilitythan do non-foamed polypropylene filaments.

The aspects of this invention which are capable of illustration, areshown in the accompanying drawings,

FIG. 1 is an end view of a single bristle made according to the presentinvention.

FIG. 2 is a schematic view of a suitable overall arrangement ofapparatus for carrying out the method of this invention.

FIG. 3 is a schematic View of a section of a modification of theapparatus shown in FIG. 2.

FIG. 4 is a schematic view of another apparatus suitable for carryingout this invention in which the filament is softened in a heatingchamber which does not contain any supporting surface in the heatingchamber itself.

FIG. 5 is a view of a section of a brush in which the bristles of thisinvention are adapted for use.

FIG. 1 is an end view of a 10% foamed mil bristle under magnification ofx. The bristle 14 has a structure such that the major part A of thefoamed portion is located at the center area of the bristle while theouter part B is continuous and non-porous. Approximately 70% of thefoamed portion is limited to not more than 30% of the center area.

In FIG. 2, a hopper it contains pellets 11 of the polypropylene andfoaming agent starting material such as crystalline isotacticpolypropylene and Kempore R125. The pellets 11 may be preheated in thehopper 10 if desired. From the hopper 1% the pellets are conveyed bymeans of a heated extrusion chamber 12 to a heated nonvented extrusionhead 13 which contains an extrusion die 13a. In the extrusion chamber12, the temperature of the mixture is raised to above the melting pointof the polypropylene and above the decomposition point of the foamingagent. It is extruded through suitably shaped orifices in the extrusiondie 13a into one or more filaments 14. A preferred extrusion temperaturefor the isotactic polypropylene is about 480 F. when the filaments 14are extruded at a linear rate of from about 18 to about 50 feet perminute from the orifices, the size of which may range from about 10 milsto about 500 mils in diameter.

In making filaments of isotactic foamed polypropylene, it isadvantageous to quench the extruded filaments in order to solidify them.Quenching at a temperature below about 60 F. imparts valuable propertiesto the drawn polypropylene as described in U.S. Patent 3,059,- 991 to P.C. Munt (the disclosure of which is incorporated herein by reference).This may be accomplished, as shown in FIG. 2, by placing a quench bath15 between the extrusion head 13 and the heating chamber 22. Theextruded filaments 14 are guided into the quench bath 15, containing aliquid non-solvent for isotactic polypylene, e.g., water, by a guideroll 16. The bath 15 is maintained in a suitable tank 17 at atemperature of 60 F. or below. Temperatures of about 40 F. arepreferred. A much lower temperature, such as 10 F., may be used for thequench bath if precautions are taken to keep the nonsolvent fromfreezing. Thus, a salt water brine at temperatures below 32 F. may beused. An immersion time of from about 8 seconds to about 20 seconds ofthe filaments 14 in the bath 15 is generally sufficient. The filaments14 are transported around a stationary pin 18 in the quench bath 15 andthen over the roll 19 and into the hot air conditioning oven 22. In theoven, the extruded filaments are transported over a number of rolls 20,which may be heated, in a sinuous or zig-zag path, as heated air iscirculated from overhead as indicated by the arrows. As the filamentspass through the heating zone,

each succeeding driven roll 20 over which the filaments 14 pass isdriven at a slightly increased peripheral speed from that or theproceeding roll so as to prevent the filaments from sagging appreciably.The primary purpose of the series of driven rolls 21 is to provide aheat exchange relaand the amounts of foaming agent (Kempore R-125)indicated in the following table are fed into a screw extruder of thetype shown in FIG. 2 having a 2.5 inch screw diameter. The die platecontains 3 extrusions orifices, each having a circular shape. Thenon-vented tionship between the filaments 14 and the heated air in 5jacket is heated to a temperature of about 480 and the the oven wherebythe filaments are uniformly softened by filaments are extruded at alinear rate of '35 feet per min heat. Since isotactic foamedpolypropylene has a softenute from the orifices. The cross sectionsofeach of the ing temperature in the range of from about 260 F. toextruded filaments are substantially round. The filaments about 305 F.,it is preferred to maintain the temperature are then passed through aquench bath maintained at a i th oven t abgut 300 F temperature of about40 F. to solidify them. They are After leaving the last and uppermostdriven roll 2%, the then fed into a heating chamber and are transportedover filaments are snubbed-With a three roll assembly 21, each a seriesof rolls. Hot air is circulated through the heatroll of which is drivenatabout the same as or a higher ing chamber, thereby heating thefilaments to a temper- ,en'pheral speed than that of the last drivenroll 20. A attire of about 300 F. before they leave the heating fastsnub roll assembly 23 is provided just outside the chamber. Afterleaving the heating chamber, the filaoven ZZZ-which is drivenat aperipheral speed of about 6 ments are stretched to about eight timestheir original to l1 times-that-of the assembly rolls 21. Thereby, thelength to give 80 mil diameter filaments. The cross secfilaments 14 arestretched from about 6 to 11 times their tions of the drawn foamedfilaments are substwtially of length. This increases the rnolecularorientation along the same shape as the cross sections of the filamentsafter the fiber axis. The drawn, oriented filaments 14 are they areextruded but before they are quenched and taken thereafter collected ona reel 24 which is supported on a over the series of rolls in theheating chamber. frame 25. The following table lists the percent blowand the FIG. 3 illustrates a modification of the apparatus shownspecific gravity as well as the measured volumetric abrain FIG. 2 inwhich extruded filaments 14 are solidified sion rate expressed in (in./min.) 10 the ratio of the by air cooling rather than by quenching. Thefilaments 29 major axis to the minor axis (D /D and the diameter 14 arepassed over the rolls 31 to allow them to solidify variation for thefilaments produced by each example. prior to being transported into thehot air conditioning As can be seen from this table, the non foamedfilament oven 22. (Example 8) is much more out of round and shows muchIn PEG. 4, filaments 4%) having a substantially round greater diametervariation than the foamed filaments (Exsectional shape prepared bymeans, previously described, amples 1 to 7). Moreover, the abrasionresistance of the are passed over a snub roll assembly 39 and thenceinto foamed filaments compares favorably with the abrasion the heatingchamber 41. A fluid heating means, such as resistance of the non foamedfilament.

Table Amount Standard Percent Foaming Volumetric Deviation VariationExtremes Example Agent. Percent Dz/D Abrasion, Specific of Diaof DiaoiVaria- Percent Blow infi/min. Gravity meter meter tion By from FromVr'eight Av. Av.

0. 01 4. 4 1. s9 0. 00055 0. s0 0. 0030 3. 75 i0. 015 0. 03 0. 7 1. s30. 00003 0. 8t 0. 0020 2. 3 i0. 010 0. 05 '1. s 1. 265 0. 00075 0. s2 0.0026 3 i0. 012 0. 0s 0. s 1. 24 0. 00070 9. s1 0. 0010 1.10 :b(). 014 0.07 11 1. 20 0. 0005s 0. s0 0. 0022 2. 5 i0. Q10 0. 00 13. 3 1.11 0.00005 0. 7s 0.0010 1.16 it). 010 0.10 15. 5 1.15 0. 00070 0. 7s 0.0012 1. 5 i0. 008 0 0 1. 450 0. 00060 0. e0 0. 0055 0 i0. 010

uperheated steam, is circulated through the heating EXAMPLE 9 Chambgr'The .fluid is introduced .into the heating cham' A series of bristlesvarying in diameters of from about bar P l i The 30 mils to about 500mils, specifically 30 mils, 250 mils temperature lfeatmg Chamber 1smamtamed at a and 560 mils, in thickness as measured after drawingtelfiper'ature sufiiflen to soften the filaments heat are extruded andquenched as described in the above filamems .are snubbed by i 3 assemolyexamples using snfficient blowing agent to achieve a 15% lmmedlatili.afteimeyfezfve l heating cnamber' These blow. The quench bath ismaintained at a temperature mus am drivel} a shghdy higher penpher.a1Speed than of about 40 F. The bristles are then fed into a heating lrate Whlch The filamnts are passed Into heatchamber where they are heatsoftened and stretch oriented. mg q F 5 keep me filaments p Saggmg Oneach foamed bristle, the ratio of the major axis to preclably Whfiefnshamber' After being snubbed by the minor axis before quenching, as inthe 80 mil bristle the roll assembly 4-4 th e softened filaments aredrawn to of Example 1, is about unity. After they are quenched, sgYeraltunes f then lgngth by a fast mu 45 Whlch 13 heat softened andstretched, there is little variation in the f at Penpheral of about 6 11or more dimensional ratio of the cross sections and little diametertunes that of the Snub mu ass'ambly 44' Th5 filaments variation. Theabrasion resistance of these bristles is good. may then be collected ona conventional reel assembly 1 claim: Such as desc'nbed m f 1. Anabrasion resistant filament consisting essentially In FIG. 5, aplurality of bristles 14 are grouped toof oriented isotacfic foamedpolypropylfim; gether at one end 5% and inserted into retaining meansthe mic of tbs di-fiersnce between the Specific gravity a hole) 5 on astreet brush base of said foamed filament and the specific gravity ofThe fonOWmg flluslrate best mode 7 the original polypropylene from whichit is formed, to templaied for carrymg out thls mvemlon: 0 the specificgravity of the original polypropylene from EXAMPLES 1 To 8 said filamentis formed having a value of up to In these examples, mixtures ofisotactic polypropylene the ratio of the length of the major axis to thelength having an aver-age molecular Weight of about 100,000, a of theminor axis of the cross section of said filadensity of 0.90 and acrystalline melting point of 333 F.,

merit being not greater than about 1.39;

*3 the diameter of said filament being substantially constant along thelength thereof; said filament having a major part of the foamed portionlocated at the center area and extending length- Wise through saidfilament. 2. A filament as defined in claim 1 having a diameter of from10 to 500 mils.

3. A filament as defined in claim 1 having a diameter of from 75 to 250mils.

4. An abrasion resistant brush bristle consisting essen- V tially oforiented isotactic foamed polypropylene;

the ratio of the ditierence between the specific gravity of said foamedbristle and the specific gravity of the original polypropylene fromwhich it is formed, to the specific gravity of the originalpolypropylene from Which said bristle is formed having a value of up to0.15;

the ratio of the length of the major axis to the length of the minoraxis of the cross section of said bristle being no greater than about1.39;

the diameter of said bristle being substantially constant along thelength thereof; said bristle having at least approximately 70% of saidfoamed portion limited to not more than approximately 30% of the centerarea of said bristle. 5. A brush containing a plurality of bristles asdefined in claim 4.

References Cited in the file of this patent UNITED STATES PATENTS2,341,823 Smith Feb. 15, 1944 2,659,921 Osborn Nov. 24, 1953 2,812,530Vfhitesel Nov. 12, 1957 2,888,415 Jankens May 26, 1959 2,905,648 HaasSept. 22, 1959 2,907,096 Halbig Oct. 6, 1959 2,913,769 Kastli Nov. 24,1959 2,948,048 Jankens Aug. 9, 1960 2,950,495 Stingley Aug. 30, 19603,050,070 Sidelman Aug. 21, 1962 3,059,991 Munt Oct. 23, 1962

1. AN ABRASION RESISTANT FILAMENT CONSISTING ESSENTIALLY OF ORIENTEDISOTACTIC FOAMED POLYPROPYLENE; THE RATIO OF THE DIFFERENCE BETWEEN THESPECIFIC GRAVITY OF SAID FOAMED FILAMENT AND THE SPECIFIC GRAVITY OF THEORIGINAL POLYPROPYLENE FROM WHICH IT IS FORMED, TO THE SPECIFIC GRAVITYOF THE ORIGINAL POLYPROPYLENE FROM WHICH SAID FILAMENT IS FORMED HAVINGA VALUE OF UP TO 0.15; THE RATIO OF THE LENGTH OF THE MAJOR AXIS TO THELENGTH OF THE MINOR AXIS OF THE CROSS SECTION OF SAID FILAMENT BEING NOTGREATER THAN ABOUT 1.39: THE DIAMETER OF SAID FILAMENT BEINGSUBSTNATIALLY CONSTANT ALONG THE LENGTH THEREOF; SAID FILAMENT HAVING AMAJOR PART OF THE FOAMED PORTION LOCATED AT THE CENTER AREA ANDEXTENDING LENGTHWISE THROUGH SAID FILAMENT.